The present invention relates to a scroll fluid machine and, more particularly, to a scroll compressor for air conditioning.
A so-called bulbous wrap has been conventionally used in which the winding-start portion, that is, the inner peripheral end of the wrap has been made thicker to increase the strength of a scroll wrap. To prevent the bulbous portion from coming into contact with the opposite wrap during operation, some of those scroll wraps have been provided with a slight relief as shown in the Japanese Patent Laid-Open Publication No. 60-252187, or U.S. Pat. Nos. 4,666,380 or 4,678,415.
As another method, there was a so-called wrap with trapezoid section, the shape of a section having large base width and small tip width throughout the wrap as shown in the Japanese Patent Laid-Open Publication No. 57-195801.
In these prior art constructions, no consideration was given to the performance though the strength was taken into consideration. Compressed gas finally collects at the winding-start portion of the wrap, which communicates with a discharge port to form a passage along which the gas flows out. Therefore, if the size of the passage is reduced due to too much emphasis being placed on the strength, the gas flowing through the passage encounters a large resistance and the compression line in the p-i diagram expands in the discharge stroke, resulting in a loss of the power. Provision of a slight relief which is large enough to prevent contact cannot solve this problem.
On the other hand, an ordinary conventional involute wrap known has a shape in which the winding-start portion is progressively thinned toward the inner periphery, that is, wedge-shaped. In this shape, a large stress due to gas pressure load occurs at the base of the wrap at the thinner inner peripheral end. According to this arrangement, although the strength is somewhat reduced, a sufficiently large passage area to the discharge port is obtained in the discharge stroke so that the flow resistance along the passage is advantageously reduced.